I think it started as some kind of CI/CD tools, then they jumped on the AI hype and they started to use it to make it possible to run agents in containers easily... perhaps to do automated actions on CI/CD pipelines that use agents (eg try to solve some minor bugs automatically when you push on a branch, etc)

Although I'm not sure if that's so much a value-added? It's not so hard to just create a container and launch an agent in it.

The whole interesting thing was to use actual programming languages for Docker build, which I think was what they initially tried to do, but now it's a bit incomprehensible... I guess conceptually Dagger relates to Dockerfile a bit like Pulumi related to Terraform?